Segmental retaining walls are commonly used in both residential and commercial applications to create usable real estate. Fencing is often required behind such walls to reduce the potential for falls and other potential hazards. In addition, guardrails usually are required in applications where parking lots or roadways are located near top of the wall.
Fence posts typically are mounted using concrete footings. A concrete footing can be created by digging a cavity in the ground, placing a bottom portion of the fence post in the cavity, and pouring concrete into the cavity.
Segmental retaining walls often include a reinforcing tie back system. For example, multiple layers of geosynthetic soil reinforcing material (commonly referred to as “geogrid”) can be secured to the wall face so that the layers extend horizontally into the surrounding stone or soil. The interaction between the stone or soil and the reinforcing material can help to stabilize the wall face, i.e., the portion of the wall formed by stacked concrete blocks.
Digging a cavity for a fence-post footing near a segmental retaining wall, after the reinforcing material has been installed, can necessitate drilling through the reinforcing material. Drilling through the reinforcing material can adversely affect the integrity thereof, and therefore is undesirable. Hence, the cavities for fence posts located near segmental retaining walls are usually created as the wall is constructed.
Fence-post cavities can be created using cylindrical cardboard forms, such as the SONOTUBE form available from Sonoco Products Company. These forms usually are provided in relatively long lengths, and therefore must be cut to a desired length at the installation site. The form is placed on the backfill material (typically soil) used behind that wall, as backfill material reaches a predetermined height. The predetermined height is chosen so that the top of the form is exposed from above ground after the wall has been completed, and all backfill material has been introduced and compacted. The form defines an open cavity in the ground that can receive the fence post.
The soil used as backfill material is usually kept moist, to help to achieve maximum density during compacting. Cardboard forms can be adversely affected by such moisture. Moisture from precipitation also can affect the integrity of a cardboard form. Also, the loads on the cardboard form resulting from the compacted backfill material, if excessive, can cause the form to collapse.
Alternatively, the form used to create the cavity can be created by cutting a predetermined length of polyvinyl chloride (PVC) or high-density polyethylene (HDPE) pipe. These materials are usually delivered to the installation site in ten or twenty-foot lengths. The need to cut the pipe creates an additional step in the construction process for the wall. Moreover, installers often cut the pipe using concrete demolition saws, chain saws, and other tooling not made for this particular use, thereby creating a potential safety hazard.
The cavity defined by the form creates a potential for injuries resulting from tripping over or stepping into an open hole in the ground. Moreover, the open cavity can fill with dirt and other debris, particularly in installations where fence posts will not be installed immediately after completion of the segmental retaining wall.
Many design codes, and many design engineers require that fence posts used near segmental retaining walls be placed at least three feet from the wall face. This requirement is intended to minimize the potential for the fence post to affect the structural integrity of the wall face. In particular, a linear force placed on the fence post, in a direction toward the wall face, has the potential to cause direct sliding of the fence post and footing toward the wall face. Such a force also introduces a moment on the fence post that can urge the fence post and footing toward the wall face. Movement of the fence post toward the wall face potentially can weaken, bulge, or overturn the wall face if the fence post is located too close to the wall face. Hence, fence posts often must be installed at least three feet from the face of a segmental retaining wall to avoid placing excessive loads on the wall face.
The real estate located between the wall face and the fence as a result of the three-foot setback requirement represents underutilized space. This area also creates a potential safety hazard. For example, individuals (and in particular, children) can fall from the setback area onto the surface in front of the wall.
The three-foot setback requirement usually places the sleeves at a location in the soil backfill behind the wall face (rather than in the crushed stone backfill used directly adjacent to the wall face.) This requirement can potentially interfere with the compacting operations performed on the backfill soil. For example, care must be exercise to avoid contacting the sleeves the equipment used to compact the soil. Moreover, the size of the compacting equipment may be limited by the need to maneuver around the sleeves.
The three-foot setback requirement also introduces the potential for the fence post to be installed too close to the wall face by mistake, in violation of design codes or site plans. In such cases, an entire fence may need to be removed and reinstalled at the proper location.